The Lorenz Energy Cycle of the Southern Ocean Mode

Abstract

Current climate models employ horizontal grid resolutions of about 1 deg in the ocean component which do not allow for the explicit representation of mesoscale processes such as eddies. It is likely that this limitation implies an underestimation of the variability of crucial climate quantities (e.g. the global mean temperature) since eddy-resolving ocean models enable specific modes of internal low-frequency variability that are not excited in non-eddy-resolving ocean models. Recently, the emergence of the so-called Southern Ocean Mode (SOM) in a state-of-the-art global eddy-resolving ocean model was described. The SOM exhibits multidecadal variability in the global ocean heat content of intense magnitude and appears to originate from interlinked dynamics of the Antarctic Circumpolar Current and neighboring gyres. In this study we continue the analysis of the SOM by exploring, for the first time, the time evolution of the mechanical energy cycle (the Lorenz energy cycle, LEC) in a realistic global eddy-resolving ocean model. We show that all globally integrated components of the LEC, that is, generation, reservoir, and conversion terms, exhibit multidecadal variability on the time scale of the SOM. The mechanical energy analysis yields further insights into the complexity of the large-scale internal ocean variability related to the SOM where several processes such as baroclinic and barotropic instabilities and mean-eddy-topography interactions turn out to be crucial.